Among PCFs which are formed of a quartz glass core, and of polymer cladding which is provided around this quartz glass core, in particular, PCF having a step index type (referred to below as an SI type) of refractive index profile have the advantage that they can be manufactured at low cost because it is not necessary for a dopant such as germanium to be added thereto. Moreover, because they have a large diameter and a high NA, they also have the feature that their coupling efficiency with a light source is excellent.
Conventionally, in order to improve characteristics such as the capacity of transmission and bending loss of an SI type of multimode fiber, a technology in which the core is formed of a multilayer structure including two or more layers has been disclosed in Patent document 1. Moreover, forming the core as a graded index (GI) type of refractive index profile is also disclosed in both Patent documents 2 and 3.
In particular, in order to improve temperature characteristics of the refractive index of the polymer cladding material of a PCF, a two-core structure is also disclosed in Patent document 3 in which fluorine-doped glass cladding is provided on a glass core, and a polymer cladding is then further provided on the outer circumference thereof.    [Patent document 1] Japanese Patent Application Laid-Open (JP-A) No. 2005-321686    [Patent document 2] Japanese Patent Application Laid-Open (JP-A) No. 11-64665    [Patent document 3] Japanese Patent Application Laid-Open (JP-A) No. 11-119036